Heat treatment apparatuses are used in fabricating semiconductor devices to heat-treat workpieces, such as semiconductor wafers, for oxidation, diffusion, CVD, annealing and the like. A known vertical heat treatment apparatus is provided with a heat treatment furnace capable of heat-treating a plurality of semiconductor wafers in an atmosphere of a predetermined process gas. This known heat treatment apparatus has a casing serving as an outer covering, and a vertical heat treatment furnace provided with an opening in a lower portion thereof and disposed in a back upper region in the casing. A loading area extends under the heat treatment furnace.
Arranged in the loading area are a transfer mechanism for transferring semiconductor wafers between a carrier, i.e., a carrying container capable of containing a plurality of semiconductor wafers, and a wafer boat, i.e., a holder for holding semiconductor wafers, and a lid lifting mechanism for vertically moving a lid for covering the opening of the heat treatment furnace, loaded with the wafer boat mounted on a heat insulating cylinder placed on the lid to carry the wafer boat through the opening into and to carry the same out of the heat treatment furnace. The casing is provided in its back part with an entrance to enable an operator to enter the loading area for maintenance and a door to close the entrance.
The heat treatment apparatus is controlled during a general process so that pressure in the loading area of the heat treatment apparatus is slightly higher than pressure in the environment surrounding the heat treatment apparatus by 3 to 5 Pa to prevent the leakage of the ambient atmosphere into the loading area.
When the heat treatment apparatus is used for a process that uses a dangerous gas, such as arsine (AsH3), as a process gas, there is a danger of unstable substances, such as the dangerous gas and particles, remaining on semiconductor wafers held on the wafer boat leaking from the loading area outside the heat treatment apparatus even if the atmosphere in the heat treatment furnace is replaced with an inert gas, such as nitrogen gas (N2). Therefore it is difficult to carry out a process that uses a dangerous gas by the heat treatment apparatus.